1. Field of the Invention
The present invention relates to an ink composition, an ink composition suitable for inkjet recording, an inkjet recording method making use of the ink composition, and a printed article obtained by the inkjet recording method.
2. Related Art
Image recording methods of forming an image on a recording medium such as paper based on image data signals, involve electrophotographic systems, sublimation type or melt type heat transfer systems, inkjet systems and the like. For example, inkjet systems may be operated with inexpensive recording apparatuses, and perform image formation directly on a recording medium by ejecting an ink composition only on the image areas where necessary. Thus, the inkjet systems may utilize ink with high efficiency at low running costs. Furthermore, the inkjet systems make less noise, and are excellent as an image recording system.
According to the inkjet systems, printing is possible not only on ordinary paper but also on non-absorptive recording media such as a plastic sheet and a metal plate. However, enhancement of the speed and image quality upon printing is becoming an important goal, and the inkjet systems have a nature that the time required for drying and curing of liquid droplets after printing greatly affects the productivity of printed articles and the sharpness of printed images.
One of the inkjet systems is a recording system making use of an ink for inkjet recording that is curable upon irradiation with a radiation. According to this method, ink liquid droplets are cured when irradiated with a radiation immediately after ink ejection or after a predefined time period, and thereby the productivity of printing is enhanced, while sharp images may be formed.
When sensitivity enhancement is successfully achieved for the inks for inkjet recording that are curable by irradiation with a radiation such as ultraviolet radiation, high curability against radiation is imparted to the inks, and numerous advantages occur, such as an enhancement of the productivity of inkjet recording, a reduction in the electric power consumption, an increase in the lifespan of the radiation generator due to decreased load, and prevention of volatilization and generation of low molecular weight substances resulting from insufficient curing. Furthermore, the sensitivity enhancement is important, particularly in terms of enhancing the strength of the formed images.
Such inkjet systems that are curable by a radiation, for example, ultraviolet radiation, are attracting an increasing attraction in recent years, since the systems produce relative less foul odor and are capable of recording on a recording medium which lacks quick-dryability and ink absorbability. Thus, there has been suggested an ultraviolet-curable ink composition utilizing radical polymerization (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 63-235382, 3-216379 and 5-214280, Japanese Patent Application Publication (JP-B) Nos. 6-21256 and 6-62905).
There is also known an ink composition having high curing sensitivity and capable of producing images with good image quality, which composition includes at least one hydrophobic polymer selected from the group consisting of a fluoro group-containing polymer, a long-chain alkyl group-containing polymer and an alicyclic group-containing polymer (see, for example, JP-A No. 2006-182970).
However, these ink compositions have problems such as the tackiness of formed images, the occurrence of the phenomenon of low molecular weight components exuding at the image surface (so-called leaching), and insufficient blocking resistance. Furthermore, if a large amount of polyfunctional monomers are used to suppress leaching, the viscosity of the ink composition increases so that images lack smoothness and there may be concerns about the jettability of the ink composition. There has also been a problem of the cured images (cured films) becoming brittle.
It has been suggested to use various photopolymerization initiators in ink compositions in order to increase the sensitivity to radiation and to enhance the blocking resistance (U.S. Pat. No. 4,134,813, JP-A Nos. 1-253731 and 6-308727), but the exposure of image during the image formation process requires a long time.
It has also been suggested to use a phosphine compound for the purpose of enhancing the curability of the image surface so as to eventually enhance the blocking resistance (JP-A No. 7-126551). However, although the effect of enhancement could be obtained, sufficient crosslinking densities were not obtained, and the blocking resistance improving effect was insufficient.
As discussed above, there is an urgent demand for an ink composition that cures with high sensitivity to radiation, has excellent blocking resistance, and is capable of forming images excellent in smoothness.